1. Field of the Invention
The present invention generally relates to a zoom lens used in an optical device, such as a still camera or a video camera, and particularly relates to the zoom lens which is of an inner focusing type or of a rear focusing type, in which a feeding amount of a focusing lens group is changed according to a zooming region where a focusing operation is performed.
2. Description of the Related Arts
A zoom lens is capable of changing a magnification of an arbitrary object, while the object is in focus. That is, when a zooming operation is performed, a plurality of lens groups which the zoom lens comprises move relatively to one another along an optical axis of the zoom lens, and a particular lens group of the lens groups performs a focusing operation as well as the zooming operation.
Hereinafter, such a lens group having the function to perform the focusing operation is referred to as a "focusing lens group," and the lens group except the "focusing lens group" is referred to as a "zooming lens group."
That is, when a distance to the arbitrary object is changed with a magnification unchanged, the zooming lens group do not move, and only the focusing lens group moves along the optical axis thereof, thus performing the focusing operation.
When the object moves from a closest position to an infinite position relative to the zoom lens with a magnification being fixed, the amount in movement required for the focusing lens group which is of a front focus type is constant irrespective of the magnification. Meanwhile, the amount in movement required for the focusing lens group which is of an inner focusing type or of a rear focusing type is not constant when the magnification is changed, i.e., when the zooming region is changed.
Accordingly, as to the zoom lens in which the zoom lens operation barrel is rotated, or is moved back and forth, manually or automatically, at time of zooming or a zooming operation, a focusing cam is adopted to change a feeding amount of the focusing lens group according to a desired zooming region with respect to a constant rotation amount of the zoom lens operation barrel or to a constant movement amount thereof along the optical axis.
FIG. 1 is a general sectional view showing an example of a zoom lens which comprises four lens groups; namely, a first lens group 1, a second lens group 2, a third lens group 3, and a fourth lens group 4. The construction of the zoom lens is generally similar to that disclosed in U.S. Patent Publication No. 5144490.
In the example, the second lens group 2 is used as the focusing lens group, whereas the first lens group 1, the third lens group 3, and the fourth lens groups 4 are, respectively, used as the zooming lens group. Paying attention to the second lens group 2, as the focusing lens group, and the third lens group 3, as the zooming lens group 3, how they work and function is described below.
FIGS. 2 through 6 are partly developed explanatory views of a zoom ring 20, a fixed cylinder 10, a focusing ring 30, an annular lens frame 40 of the second lens group 2, and an annular lens frame 50 of the third lens group 3.
In the zoom lens, the fixed cylinder 10 which is stationary relative to the camera body to which the zoom lens is connected, inserts and fits in the inner periphery of the zoom ring 20, and the focusing ring 30 inserts and fits in the inner periphery of the fixed cylinder 10. When a zoom lens operation barrel 5 which is positioned outermost is rotated at time of the zooming operation, the zoom ring 20 rotates in unison with the rotation of the zoom lens operation barrel. The focusing ring guide pin 31 which is fixed to the focusing ring 30 is inserted into both zoom cam slot 11 of the fixed cylinder 10 and guide slot 21 of the zoom ring 20. The guide slot 21 functions to linearly guide the focusing ring 30. Thus, upon rotation of the zoom ring 20, the focusing ring 30 moves back and forth, with the focusing ring 30 rotating.
On the other hand, the annular lens frame 40 of the second lens group 2 inserts and fits in the inner periphery of the focusing ring 30. A second lens group guide pin 41 which is fixed to the annular lens frame 40 is inserted into a focusing cam slot 32 of the focusing ring 30. Accordingly, the second lens group 2 moves back and forth by an amount corresponding to the amount in movement of the focusing ring 30 in the direction of the optical axis.
Provided that the amount in movement of the second lens group 2 is Z.sub.1, the amount Z.sub.1 in the movement is determined by a component in the direction of the optical axis of the zoom cam slot 11 formed on the fixed cylinder 10.
The second lens group guide pin 41 penetrates through an escaping hole formed on each of the fixed cylinder 10 and the zoom ring 20, and the second lens group guide pin 41 projects outside from the zoom ring 20 and engages a fork-shaped restriction member 60. The restriction member 60 is rotated relative to the fixed cylinder 10 only at time of the focusing operation, and the restriction member 60 remains stationary at time of the zooming operation. In FIG. 1, a reference numeral 61 denotes a motor unit for driving the restriction member 60.
In the construction, when the focusing ring 30 rotates at time of zooming, the second lens group guide pin 41 which engages the restriction member 60, moves along the focusing cam slot 32 of the focusing ring 30. As a result, the second lens group 2 moves back and forth relative to the focusing ring 30 by an amount which corresponds to a component in the direction of the optical axis of the focusing cam slot 32 of the focusing ring 30. The amount in movement of the second lens group 2 is Z.sub.2.
As described above, when the zoom ring 20 rotates, the focusing ring 30 moves back and forth by the amount Z.sub.1 in movement relative to the fixed cylinder 10, and the annular lens frame 40 moves back and forth by the amount Z.sub.2 in movement relative to the focusing ring 30. Consequently, the second lens group 2 moves by a total amount of (Z.sub.1 +Z.sub.2) relative to the fixed cylinder 10.
By the way, as indicated by the curving shape of the focusing cam slot 32 in FIG. 4, the amount Z.sub.2 in movement of the second lens group 2 relative to a constant amount in movement of the restriction member 60, is not constant according to its zooming region.
Meanwhile, the third lens group guide pin 51 which is fixed to the annular lens frame 50 of the third lens group 3 is inserted into both zoom cam slot 22 of the zoom ring 20 and guide slot 12 of the fixed cylinder 10. Thus, when the zoom ring 20 rotates, relative to the fixed cylinder 10 the third lens group does not rotate and moves back and forth.
Next, a description is made below on a case in which a distance to an object changes with a magnification (zooming region) not changed and constant.
The restriction member 60 is rotated relative to the fixed cylinder 10, in unison with an operation of a manual or automatic focusing (AF) mechanism. At this time, the focusing ring 30 is stationary relative to the fixed cylinder 10; therefore, the second lens group guide pin 41 is driven along the focusing cam slot 32 of the focusing ring 30 by the restriction member 60, and the annular frame 40 (therefore, the second lens group 2) moves back and forth relative to the fixed cylinder 10 by the amount Z.sub.2 corresponding to the component in the direction of the optical axis of the focusing cam slot 32, so as to perform the focusing operation. As described above, the amount Z.sub.2 in the movement changes according to the zooming region.
Although the driving mechanisms for driving the first lens group 1 and the fourth lens group 4 are not described, these lens groups 1 and 4 are moved along the optical axis only at time of the zooming operation, and the lens groups 1 and 4 are not moved at time of the focusing operation. Namely, it is only the second lens group 2 that is moved along the optical axis at time of the focusing operation.
In the above example of the conventional mechanism shown in FIGS. 1 through 6, the focusing lens group (i.e. the second lens group) 2 moves by the amount Z.sub.1 +Z.sub.2 relative to the fixed cylinder 10 along the optical axis in the zooming operation, whereas the focusing lens group 2 moves by the amount Z.sub.2 relative thereto in the focusing operation. The zoom cam slot 11 which determines the movement amount Z.sub.1 along the optical axis is formed on the fixed cylinder 10, whereas the focusing cam slot 32 which determines the movement amount Z.sub.2 along the optical axis is formed on the focusing ring 30 which is a member separate from the fixed cylinder 10.
The reason why the zoom cam slot 11 and the focusing cam slot 32 are formed on such separate members is as follows.
Namely, the reason is that in the zooming operation, not only the second lens group (i.e. the focusing lens group) 2 but also the zooming lens groups 1, 3, 4, move along the optical axis, whereas in the focusing operation, only the focusing lens group 2 moves along the optical axis. In other words, it is necessary to prevent the zooming lens groups from moving when the focusing lens group is driven at time of the focusing operation. Therefore, the focusing ring 30 has no slots other than the focusing cam slot 32 which determines the movement amount Z.sub.2 of the focusing lens group in the focusing operation.
That is, according to the conventional mechanism, the focusing ring 30 becomes a member which is exclusively used for driving the second lens group 2 at time of the focusing operation, thus increasing the number of assembling parts and increasing the size of the zoom lens as a whole.
There have been proposed a large number of conventional arts about the arrangement and configuration of the cam slot, in addition to the conventional art described above with reference to FIGS. 1 through 6. In each of the conventional arts, an annular member is adopted to drive only the focusing lens group for the focusing operation. On the other hand, in the zoom lens disclosed in each of Japanese Laid-Open Patent Publication Nos. 4-184404 and 4-184405, a cam for driving the focusing lens group is formed on a cam ring on which the cam for driving the zooming lens groups is formed.
But, in the mechanism, an annular member as a connection cylinder is provided separately. Therefore, in respect of an aspect of the number of assembling parts, the mechanism disclosed in the two publications is not so different from the mechanisms using the annular member which is adopted to drive only the focusing lens group.